The present invention relates to a culture device of a domed shape, a conical shape, or a cylindrical shape used in culture of photosynthetic organisms such as microalgae or the like, a gas discharge device disposed so as to be movable in the culture device and functioning to supply gas necessary for the culture into a culture solution and agitate the culture solution, or a culture system as a combination of the culture device with the gas discharge device.
In order to produce useful substances such as vitamins, amino acids, pigments, proteins, polysaccharides, fatty acids, and so on or in order to dispose of carbon dioxide which is considered to be one of causes of global warming, extensive research has been conducted heretofore on mass culture of microorganisms including microalgae such as Chlorella, Spirulina, or the like, and products of cultures based on the result of research are commercially available.
Most of the algae among these microorganisms absorb carbon dioxide to biosynthesize the useful substances by photosynthesis. In this case, because it is important to effect the culture of algae efficiently, a culture apparatus for making the algae efficiently perform the photosynthesis is necessary. Therefore, improvement in the conventional culture apparatus and development of new culture apparatus are under way.
The conventional algae culture apparatus commonly known include, for example, culture ponds, raceway culture devices, tubular culture devices, liquid membrane forming culture devices, and so on. The artificial culture ponds are of a type in which a culture pool or a culture tank is constructed, for example, of concrete outdoors, the culture solution is poured into the pool to form a culture pond, and the microalgae such as Chlorella or the like are cultured in the solution by making use of the sunlight. However, the systems of this type necessitate the surface area of the pool, for example, of 3000 M2 and are thus normally huge.
In addition, when the microalgae are cultured in the system of this type, concentrations of the microalgae increase in the culture solution with progress of culture, so as to turn the solution into deep green and thus inhibit the sunlight from reaching the bottom part of the culture pond. From this phenomenon, there will arise a problem that the overall efficiency of photosynthesis of algae is decreased, unless the culture concentrations of the microalgae are lowered.
For this reason, the depth of the solution has to be kept below 15 cm and broad areas are necessary for the volume culture of microalgae. Since the concentrations of the culture solution cannot be high, there arises a problem that, for collecting the cultures from the solution, the cultures must be collected from an enormous amount of the culture solution with low concentration.
On the other hand, the culture ponds have to be agitated to facilitate the photosynthesis of the microalgae, but a lot of energy is necessary for agitating the huge amount of the solution of low concentration. Further, because the culture ponds are set outdoors and are open to the air, impurities of dirt, dust, etc. are mixed readily into the solution, and microorganisms and other algae floating in the air are mixed into the ponds to propagate; these pose another problem that the cultures cannot be obtained in high purity and with high quality.
Since the culture ponds are set outdoors, the temperature varies with variations in climate and it is thus very difficult to keep the temperature of the ponds constant. Particularly, the culture ponds have such a drawback that the temperature becomes too low in the winter season, depending upon regions.
For these reasons, the culture of algae making use of the culture ponds has such a drawback that it cannot be applied to the algae except for those such as Chlorella, Spirulina, and Dunaliella which can grow even under such a special condition as high pH or high salinity.
The raceway culture apparatus is constructed in such structure that the inside of a culture tank is partitioned with straightening plates to form a circuit path of the culture solution and the algae are cultivated by a method for circulating the culture solution in the circuit path by circulating means. This method is an improvement in the method of the culture ponds, but it also fails to utilize the light efficiently, because rates of photosynthesis of algae are lowered with progress of the culture, as in the case of the culture method with the culture ponds. This also raises a problem of low utilization efficiency of carbonic acid gas. For accomplishing efficient utilization of light, there is also a suggestion of guiding the sunlight through optical fibers into the solution. (Japanese Laid-open Utility Model Application No. 5-43900)
However, since the solution is circulated by mechanical agitation in the case of the culture of algae by this method, this method will encounter such an inevitable drawback that cells of algae are subject to breakage or shear stress (a phenomenon in which the algae are cut by shearing stress to degrade the activity of cells and thus make growth rates slower).
The tubular culture apparatus is an apparatus for culturing the microalgae etc. by use of the culture tank constructed of a light-transmitting tube. When the algae are cultured by use of this apparatus, there is no contamination of the culture solution due to various germs or the like and the culture concentrations can also be high; therefore, this is an extremely advantageous method for separating the algae from the culture solution and collecting the useful substances produced by the algae.
However, after long-term culture of algae, the algae attach to the internal wall of the tube, so as to considerably decrease the quantity of light transmitted by the tube. This phenomenon makes the efficient culture of microalgae difficult and it is not easy to remove the algae attaching to the internal wall of the tube.
For solving this problem, there is a suggestion about a method for putting cleaning balls in the tube and always circulating these balls with the culture solution, thereby cleaning the internal wall of the tube (Japanese Laid-open Patent Application No. 6-90739). This method, however, has many problems; it is not possible to continuously remove the dirt and the attachment of algae on the internal wall of the tube well, the balls have to be collected and cleaned, the balls always have to be circulated in the tube, and so on. A further problem of the culture of algae according to this method is that oxygen gas resulting from the photosynthesis of algae stays inside the tube because of the culture inside the tube and this oxygen acts to inhibit the photosynthesis of algae conversely (inhibition of photosynthesis). There is thus another suggestion about an idea of a device for suppressing the adverse effect on the culture due to the oxygen evolving in the photosynthesis. (Japanese Laid-open Patent Application No. 9-121835)
The liquid film forming culture apparatus is one constructed in such structure that a light-transmitting domed lid is placed on the culture tank, the culture solution is sprayed from below toward the internal surface at the top of the domed lid to form a liquid film of the culture solution on the internal surface of the lid, and this liquid film is exposed to the light. (Japanese Laid-open Patent Application No. 8-38159)
This suggested method, however, has problems that it requires a circulating pump for continuously forming the liquid film, that it is not suitable for the mass culture, that the sunlight cannot be utilized, and so on.
Since the microalgae accumulate the useful substances in their body through the photosynthesis, a significant challenge is to make the microalgae conduct the photosynthesis at as high efficiency as possible. Conceivable factors for the efficient photosynthesis are enlargement of the light-receiving area of the culture apparatus, efficient agitation of the culture solution, adjustment of the thickness or the depth of the culture solution, easiness of cleaning to remove the microalgae attaching to the internal surface of the culture apparatus, control of temperature, prevention of mixture of foreign germs, other microalgae, and impurities, and so on.
The issue of the light-receiving area is influenced by how large the light-receiving area is or by how efficiently the culture solution is exposed to the light.
For example, in the case of the culture tanks or the culture ponds, the surface area is determined by the surface of the culture tanks or the culture ponds; therefore, the enlargement of the surface area can be achieved only by increasing the size of the tanks or the ponds and there is no alternative means.
The agitation of the culture solution is essential to uniform irradiation of light to the culture solution and an ordinary means therefor is often agitation or movement of the solution, for example, by a pump, or mechanical agitation in the tanks, the ponds, and so on.
Such mechanical agitation, however, is not preferable, because it causes breakage or shear stress of cells of microalgae.
Since photosynthesis rates differ depending upon kinds of microalgae, it is necessary to employ different depths of the culture solution between algae of low rates and algae of high rates. It is also necessary to change the depth according to an expected culture concentration. As described, the thickness or depth of the culture solution needs to be freely adjusted according to the conditions including the kind of microalgae, the expected culture concentration, and so on.
The removal and cleaning of the microalgae attaching to the internal surface of the culture apparatus is not so significant in the case of the open type culture ponds and culture tanks set outdoors. This removal and cleaning is, however, essential to the culture apparatus of the closed type, because the attaching microalgae come to intercept the light. In addition, the apparatus needs to be constructed in such structure that at the stage of completion of the culture the inside surface of the apparatus can be cleaned for the next culture, so as to remove the attachments readily.
The temperature control is very important, particularly, for the closed type apparatus, because the temperature of the solution becomes too high in the summer season and causes a trouble in the culture. One of solutions to it is a method for mixing cool water into the culture solution, but, because of dilution of the culture solution, a large amount of the culture solution thus diluted has to be dealt with in the next step of collection of the cultured algae. This method is, therefore, very disadvantageous from the industrial aspect.
The culture devices are normally either those for outdoor use or those for indoor use. Therefore, if the devices for outdoor use are used indoors, there will arise a problem that the efficiency of utilization of light is low; on the other hand, there also arises a problem that the devices for indoor use cannot be used outdoors. There are thus increasing demands for the culture apparatus of simple structure that permits the culture under ordinary culture conditions both indoors and outdoors.
The agitation of the culture solution is an essential operation to uniform culture. The reason is that it is carried out for the following purposes; (1) to cancel the difference of culture rates appearing between the surface layer part and the deep layer part of the liquid medium; (2) to uniformly distribute the gas such as air, carbon dioxide, or the like across the whole of the liquid medium or the culture solution; (3) to uniformly distribute the light over the microalgae to be cultured; (4) to prevent the microalgae likely to form colonies during the culture from settling and depositing in the liquid bottom part and to make them re-scattered in the culture solution; and so on.
It is thus necessary to always agitate the culture solution and to supply the necessary air or carbonic acid gas or the like into the culture solution.
An object of the present invention is, therefore, to provide a closed type culture device of microalgae as a culture device overcoming the drawbacks of the conventional open type or closed type culture devices, of which the shape is domed, conical, or cylindrical and which has the advantages including the following: (1) the foreign germs and impurities are prevented from being mixed in the device; (2) the temperature control of the culture solution is easy; (3) the solution can be agitated without mechanical agitation of the culture solution, so as to prevent the cells of algae from being broken and prevent the shear stress from appearing; (4) the culture concentrations can be set high; (5) the cleaning of the device is easy; (6) the culture is not inhibited by the evolving oxygen; and (7) the efficiency of utilization of light is high.
Another object of the present invention is to provide a gas discharge device used in the culture device, which has advantages including the following; when the necessary gas is supplied into the culture solution by use of this device, the device undergoes movement to agitate the solution and the solution is also agitated by the discharged gas, whereby contact is extremely good between the supplied gas and the culture solution, thereby increasing the culture efficiency.
Further, the present invention relates to a culture apparatus characterized by a combination of the above-stated culture device with the above-stated gas discharge device.
A culture device of microalgae according to the present invention is a culture device of either shape selected from a domed shape, a conical shape, and a cylindrical shape; the culture device of the domed shape is a culture device of a domed shape comprising an outside semispherical dome of a transparent material, an inside semispherical dome of a transparent material, and a bottom portion connecting lower ends of the two domes, wherein a cylindrical opening portion is provided at top part of the outside semispherical dome, and an introducing member of air and/or carbonic acid gas and a discharging member of a culture solution are provided in the bottom portion, and wherein a member for water sprinkling is provided outside the cylindrical opening portion and a sprinkled water receiver is provided around the outside periphery of the bottom portion as occasion may demand;
the culture device of the conical shape is a device comprising an outside conical peripheral wall of a transparent material, an inside conical peripheral wall of a transparent material, and a bottom portion connecting lower ends of the two peripheral walls, wherein a cylindrical opening portion is provided at top part of the outside conical peripheral wall, and an introducing member of air and/or carbonic acid gas and a discharging member of a culture solution are provided in the bottom portion, and wherein a member for water sprinkling is provided outside the cylindrical opening portion and a sprinkled water receiver is provided around the outside periphery of the bottom portion as occasion may demand;
the culture device of the cylindrical shape is a device comprising an outside cylindrical peripheral wall having an upper wall of a transparent material, an inside cylindrical peripheral wall having an upper wall of a transparent material, and a bottom portion connecting lower ends of the two peripheral walls, wherein a cylindrical opening portion is provided in central part of the upper wall of the outside cylindrical peripheral wall, and an introducing member of air and/or carbonic acid gas and a discharging member of a culture solution are provided in the bottom portion, and wherein a water sprinkling member is provided outside the cylindrical opening portion and a sprinkled water receiver is provided around the outside periphery of the bottom portion as occasion may demand.
A gas discharge device for use in the culture device of microalgae is a device comprising two opposed rectangular base plates, a bubble guide member of a U-shaped cross section or an inverted-U-shaped cross section opening down, and a discharge nozzle, wherein the bubble guide member is set as inclined with respect to upper surfaces of the rectangular base plates, an inclined wall as an upper surface thereof is bent at an upper end thereof to form an upper wall extending substantially horizontally, the bubble guide member has side walls hanging down from the both side edges of the inclined wall and upper wall, and lower ends of the two side walls are joined to the upper surfaces of the rectangular base plates,
wherein the discharge nozzle is set through a through hole bored in a lower portion of the inclined wall so as to be rotatable, wherein at least one of the opposed rectangular base plates is bent in the same direction at a front end portion and/or at a rear end portion as occasion may demand, or wherein at least one of the rectangular base plates is provided with weight adjusting means.
Further, the present invention is a culture apparatus as a combination of the aforementioned culture device with the above gas discharge device.
The transparent material used in the culture devices can be any transparent material as long as it is excellent in the light-transmitting property and has sufficient weather resistance and resistance to ultraviolet rays; for example, it can be selected from materials such as acrylic resin, polycarbonate, polypropylene, polyethylene, polyvinyl chloride, glass, and so on; the synthetic resin can be suitably applicable in terms of ease to work; particularly, the acrylic resin is a most preferable material, because it has the aforementioned characteristics.
The gas introduced into the culture device must contain carbonic acid gas in its components and may be one with an increased concentration of carbonic acid gas by mixing carbonic acid gas in air; the air and carbonic acid gas may also be introduced separately into the device.
The carbonic acid gas is most preferably used in the form of a mixture with air. While an air bubble containing carbonic acid gas rises to the surface with agitating the culture solution, the carbonic acid gas is dispersed and absorbed in the culture solution and the air functions to remove oxygen evolving in the culture from the culture solution. If the carbonic acid gas is introduced alone into the culture solution introducing rates of carbonic acid gas will be low and dispersing rates of carbonic acid gas into the culture solution will tend to become slow inevitably.
The cylindrical opening portion works to exhaust the air introduced into the culture solution, unused carbonic acid gas, and evolving oxygen into the atmosphere; however, if the opening portion is open to the atmosphere, the impurities of dust etc. will enter the device. In order to prevent the mixture of such substances, it is preferable to provide the opening portion with a filter member or to provide the opening portion with such a lid member as to act in similar fashion to the filter member.
This opening portion may be molded in an integral form with the outside semispherical dome, the outside conical peripheral wall, or the upper wall of the outside cylindrical peripheral wall or may be one molded separately and fixed thereto.
The culture device of the domed shape, the conical shape, or the cylindrical shape used as a culture device body can be constructed in either of the following ways; each of the outside member and inside member is formed integrally; one of them is formed integrally while the other as an assembly of properly divided members of two parts or of four parts; the both members may be assemblies of divided members. The way of making the device can be determined according to the size and shape of the culture device.
Further, the material and structure of the sprinkled water receiver can be determined arbitrarily as long as the member can receive water streams sprinkled over and flowing down on the outside surface of the device. The material can be either a metal material or a plastic material.
The structure of the sprinkled water receiver can be either of the following; it is molded as a separate member from the culture device body; the receiver is constructed by extending the lower end of the outside member of the culture device horizontally around the outside periphery and bending the end portion upward; the sprinkled water receiver is constructed by extending the lower end of the inside member of the culture device horizontally around the outside periphery and bending the end portion upward.
A preferred configuration of the sprinkled water receiver is a member molded as a separate member from the culture device body.
The introducing member of air and/or carbonic acid gas provided in the bottom portion can be a tubular member having a lot of gas discharge ports, or gas discharge ports perforated in the bottom portion.
Since the gas introduced from this introducing member into the culture solution agitates the culture solution with moving up in the culture solution, the culture solution does not have to be mechanically agitated intentionally. This method can prevent the breakage of cells and the occurrence of shear stress due to the mechanical agitation accordingly.
The oxygen gas evolving in the photosynthesis can be discharged efficiently and quickly from the culture solution with the ascent of the gas.
There are two methods for supplying the culture solution into the culture device. The first method is a method for providing the bottom portion with a supply member (for example, a supply hole bored in the bottom portion) and supplying the culture solution through this supply member.
The second method is a method for supplying the culture solution through the cylindrical opening portion at the top part.
Provision of various supply and introducing members in the device will cause complication of the device and also have a problem of contamination on the occasion of change of the kind of microalgae to be cultured.
Therefore, the second method is most preferable.
Since the outside member and the inside member of the culture device both are made of the transparent material, if an artificial light source is provided in the inside space of the culture device the culture can also be carried out during the nighttime in the case of the outdoor culture. In the case of the indoor culture, efficient, continuous culture can be realized with two artificial light sources set inside and outside the culture device.
Since the culture device of the domed shape has a small occupying area but a large surface area, the light-receiving area thereof is large. In this device agitation of the culture solution is effected extremely well. When this device is made of a plastic material, it can be formed readily by vacuum forming and thus can be made at the lowest cost.
For these reasons, the culture device of the domed shape is most preferable as a microalgae culture device.
It is preferable to provide the culture device with various sensors such as a temperature sensor, a liquid level sensor, a pH sensor, a dissolved oxygen amount sensor, or the like for controlling and monitoring the culture conditions. These sensors are set through the cylindrical opening portion or through the outside wall of the device.
Since the gas discharge device of the present invention discharges the gas of air or the like obliquely downwardly toward the bottom portion of the culture device, it advances as hopping like a frog in the device. This motion causes the culture solution to be agitated hard and the discharged gas also agitates the culture solution during the ascent in the culture solution. Particularly, in cases where the microalgae to be cultured are likely to form colonies, the gas discharged from the gas discharge device breaks the colonies and disperses the microalgae in the culture solution, thereby increasing the culture efficiency.
The gas discharge device is normally made of a plastic material and is provided with the weight adjusting means to adjust the weight of the device.
The microalgae that can be cultured by the use of the culture device of the present invention include a variety of microalgae, in addition to Chlorella, Spirulina, and Dunaliella commercially successful heretofore. For example, such microalgae include those with excellent values added that produce useful substances, such as Haematococcus pluviaris producing xcex2-ketocarotenoid (astaxanthin), Isochrysis galbana (Isocrysis galvana) being used as live bait for cultivation of marine fishes or producing a highly unsaturated fatty acid (DHA), Nannochloropsis oculata also being used as live bait for cultivation of marine fishes or producing a highly unsaturated fatty acid (EPA), and so on. The culture apparatus of the present invention can culture these microalgae in high concentrations and at high utilization efficiency of light.